Hi Madhavan,

Each of export type will have their own behaviour. As far I know, export in PDF has a different size layout in excel. In Excel the layout can be better because Excel already provides the table,cell and row. The width also fit in Excel Layout. 

While in PDF, the layout width is narrow/limited. The export process would be decide the best fit based on the available layout.
That's why there is a multiline header if the text in header is too long. The column 'Priority' and 'Response By' looks so close because the alignment you set. This also happened because Pdf layout has less UI than Excel (PDF doesn't provide table,cell and row layout like Excel). 

Note:Default Behaviour

Regards,
Handy